Recently, miniaturization and a high-performance of electronic apparatuses such as a personal computer, a cellphone, and the like have advanced rapidly, and as a power source for those, a demand for a small and light secondary battery having a high energy density is getting higher. Under such circumstances, a lithium-ion secondary battery having large charge and discharge capacity is widely used.
Conventionally, the positive electrode active material of the lithium-ion secondary battery contains cobalt as a main component. However, cobalt is a rare metal and expensive.
Therefore, attention is made to a positive electrode active material containing nickel as a main component. The positive electrode active material containing nickel as a main component contains low cobalt content compared to the positive electrode active material containing cobalt as a main component so as to have low cost. However, in the positive electrode active material containing nickel as a main component, there are present many lithium compounds such as lithium hydroxide, lithium carbonate, and the like, which are by-products and the like in an unreacted residue or at a time of synthesizing, on a particle surface or between primary particles.
Generally, in a case wherein a positive electrode active material for a lithium-ion secondary battery containing a large amount of lithium carbonate is used as a positive electrode material of a battery, the lithium carbonate decomposes, and produces carbon dioxide inside the battery so as to increase a pressure inside the battery to cause the bulging of the battery, or reduce battery characteristics such as charge/discharge efficiency and the like. Moreover, in a case wherein a positive electrode active material for a lithium-ion secondary battery containing a large amount of lithium hydroxide is used as the positive electrode material of the battery, gelation of a positive electrode paste is induced, so that a process of applying the positive electrode paste becomes difficult.
As for a method for solving the aforementioned problem, there are proposed a method for cleaning the positive electrode active material with water or an aqueous solution in which lithium is dissolved (Patent Document 1); a method for cleaning the positive electrode active material with an aqueous solution having pH 7 or above such as ammonia water, an aqueous lithium hydroxide solution, and the like (Patent Document 2); and some other methods for a cleaning treatment using various solutions.
However, even if the cleaning treatment of the positive electrode active material for a lithium-ion secondary battery is carried out by the conventionally proposed methods, the removal of lithium hydroxide or lithium carbonate, which is unsuitable for the positive electrode material contained in the positive electrode active material, is not necessarily sufficient.
Additionally, recently, the production quantity and usage of the lithium-ion secondary battery are expanding (for example, a storage battery for a solar power generation, a battery for an electric power source for an electric vehicle, an airplane, and the like), so that a large quantity of positive electrode active materials is required, and as a result, there arises a new problem that a large quantity of waste fluid generated in a production process of the positive electrode active materials has to be treated.